ci: use go modules when installing gnmi_target

Author: abn

.github/workflows/main.yml

@@ -61,8 +61,7 @@ jobs:
       - name: Install gnmi_target
         shell: bash
         run: |
-          go get -u github.com/google/gnxi/gnmi_target
-          go install -v github.com/google/gnxi/gnmi_target
+          GO111MODULE=on go get github.com/google/gnxi/gnmi_target
           echo `go env GOPATH`/bin >> $GITHUB_PATH
 
       - name: Run tests